The present invention relates to a radio relay for transmitting single side band (SSB) multiplex signals, and particularly to such radio relay for reducing interference crosstalk between the transmission signals.
An SSB super-multiplex telephone signal transmission is known to occupy the smallest necessary frequency bandwidth in principle. However, the super-multiplex signal transmission has been developed solely relying on FM, mainly because a superwide band amplifier having excellent linearity has been very difficult to provide in the microwave band. However, since the experimental results were published that the principle of the feed-forward amplifier invented by Harold S. Black is applicable to the microwave region (H. Seidel: "A Microwave Feed-Forward Experiment" BSTJ, Vol. 50, No. 9, pp. 2879-2916, 1971), practical research investigations for a microwave SSB super-multiplex communication system have been rapidly progressing.
If the transmission of signals having the same degree of multiplexing is intended, taking advantage of the SSB communication system over the FM communication system, the following points should be taken into consideration:
(A-1) The occupied bandwidth is halved, and PA1 (A-2) An echo distortion noise inherent to an FM communication system is not caused. PA1 (B-1) It is necessary to suppress to the order of a several Hz the carrier frequency variation of SSB multiplex signals caused by the relaying, and PA1 (B-2) It is necessary to reduce the interference crosstalk between transmission signals from the first terminal station to the second terminal station (hereinafter referred to as "down link") and those in the opposite direction (hereinafter referred to as "up link") that is determined by the front-to-back coupling ratio of antennas. PA1 (C-1) In a terminal station receiver, it is necessary to suppress this dispersal modulation signal to about 2 Hz peak-to-peak, and so a negative feed-back circuit having an extremely high compression factor is required. PA1 (C-2) In a microwave communication system, when a large number of repeater stations are cascaded, an IF switching station is needed to increase reliability of the system. In this case, since the frequencies and phases of the dispersal signals of the operating channel and the stand-by channel are generally not the same, there is a disadvantage that a frequency deviation of the SSB multiplex signals is caused upon switching at the IF switching station and thus the frequency of the SSB multiplex signals would vary largely until an AFC completes its follow-up control.
On the other hand, however, the SSB communication system involves the following problems:
The SSB communication system in the microwave frequency band, is well known in a coaxial cable transmission system, required an extremely excellent frequency stability for satisfying the condition (B-1). Accordingly, as a repeater for an intermediate repeater station, it is desirable to use a repeater employing a local oscillator of shift frequency converter type which can precisely maintain the difference between the receiving frequency and the transmitting frequency in a relatively easy manner. On the other hand, as transmitting and receiving repeaters for a terminal station, a local oscillator having a frequency stability of the order of 10.sup.-9 is necessitated. However, a practical oscillator satisfying this condition is difficult to obtain. As a solution to this problem, a method is used for controlling the oscillation frequency of the receiving local oscillator in the receiver of a terminal station so that a continuous pilot signal (for instance, 8,500 KHz) contained in the SSB multiplex signal may become just a predetermined frequency at the demodulation output of the receiver.
By employing the aforementioned method, it is possible to limit to a region of several Hz the frequency deviation caused by the relaying.
Further, in a first and a second antennas in common use for reception and transmission at a repeater station (as shown in FIG. 1-6 and described on pp. 1-19 of Philip F. Panter: Communication Systems Design, McGraw-Hill, 1972) of an up link and a down link, since the receiving signals at the first and second antennas (S.sub.1 and S.sub.2 of 5945.2 MHz, for example, in a 6 GHz-band corresponding to f.sub.1, f.sub.3 or f.sub.5 shown in FIG. 1-6 of the text by Panter) and the transmitting signals at the first and second antennas (for example, S.sub.1 ' and S.sub.2 ' of 6197.2 MHz corresponding to f.sub.7, f.sub.9 or f.sub.11 shown in FIG. 1-6 of the text by Panter) have their carrier center frequencies selected substantially equal to each other, there exists an interference that is determined by a front-to-back ratio of the first and second antennas. In general, the magnitude of the front-to-back ratio is determined by the type and the aperture area of the antenna and the relative angle between antennas, and it is of the order of 60 dB .about. 70 dB. Normally, the magnitudes of S.sub.1 and S.sub.2 (or S.sub.1 ' and S.sub.2 ') are substantially at the same level, but since they are frequency-modulated under a noise-loaded condition, it has been known that a signal-to-noise ratio is improved by 16 dB with respect to a signal-to-interference ratio owing to the so-called dispersal effect. The detailed description is given in Yonezawa et al: Microwave Communication, Maruzen Co., 1963 at pp. 374-376.
Accordingly, in an FM communication system, the degradation of the signal-to-noise ratio that is determined by the front-to-back ratio of antennas amounts to 76 dB .about. 86 dB in one combination; this being equivalent to 14 pico-watts to 1.4 pico-watts when converted into psophometric noise power, and so it is seen that this value is a sufficiently small value in comparison to a limit value for a permissible psophometric noise power of 100 pico-watts per one repeater.
However, in an SSB communication system, since the dispersal effect resulting from frequency modulation in the FM communication system does not exist, there still remains a problem that the signal-to-interference ratio becomes equal to the signal-to-noise ratio, and so this solely exceeds the limit value for a permissible psophometric noise power per one repeater.
As one solution to this problem, it has been proposed to utilize the aforementioned dispersal effect as described on page 374 of the text by Yonezawa et al. More particularly, it is a method in which a triangular dispersal signal is applied to a local oscillator in a terminal station transmitter and thereby frequency modulation of the order of peak-to-peak 100 KHz is applied. However, in this case the following disadvantages are involved: